1. Field of the Invention
The invention relates to a sleep control device that controls the sleep depth level of an individual person, and to a control method for the sleep control device.
2. Description of the Related Art
There has been known a sleep control device that induces a vehicle driver to sleep in the case where the wakefulness level of the driver is lowered to prevent the driver from dozing while driving (see Japanese Patent Application Publication No. 07-108847 (JP-A-07-108847), for example). The sleep control device induces the driver to sleep which utilizes natural sleep rhythms, or wakefulness rhythms, in the case where the wakefulness level is lowered to relieve the drowsiness and recover the fatigue of the driver. The wakefulness rhythms, also referred to as ultradian rhythms, include rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep alternated at a frequency of 90 minutes.
The sleep control device can help both relieve the drowsiness and recover the fatigue of an individual person. However, since the individual person is always induced to sleep which utilizes natural sleep rhythms irrespective of the drowsiness level, the individual person may not always feel refreshed when he or she wakes up. Thus, there has been desired a technique allowing the individual person to sleep in such a manner that matches the drowsiness level.